Rapid, small-volume, flow injection analysis for SCO2, and NH4+ in marine and freshwaters

∑CO2 and NH4+ in natural waters can be rapidly (∼60 samples h−1), precisely (∼1% RSD), and simply determined in small samples (20 µl, ∑CO2; 50 µl, NH4+) with an inexpensive flow injection analysis system (FIA). A gas-permeable membrane is used to remove CO2 or NH3 from acidic (∑CO2) or basic (NH4+) reagent streams into a receiving stream and conductivity detector. Linear negative responses occur over a wide range ofconcentrations (<0.1–20 mM ∑CO2; 0.1–100 µM NH4+). Adding Zn2+ to sulfidic samples prevents H2S interferences for CO2 and allows determination of H2S at high concentration (>0.1 mM) by difference (with, without Zn). Interferences from fatty acids (∑CO2) and methylamines (NH4+) are unlikely at natural levels. These methods correlate well with traditional techniques.